An electronic device in which a main microcomputer and a sub microcomputer are installed is conventionally known.
In such a conventionally known electronic device, the sub microcomputer is rewritten by using a rewriting tool (an external device). Specifically, reprogramming data for the sub microcomputer is transferred from the rewriting tool to the main microcomputer. The main microcomputer transfers the transferred reprogramming data to the sub microcomputer. The sub microcomputer rewrites a program in the sub microcomputer by using the transferred reprogramming data.